In the biomedical field, knowledge of the functional status generally is at least as informative as, and often is more informative than knowledge of the structural status of tissues and organs in determining the pathological state thereof.
In the past, physiological and pathophysiological interrogation of biological targets has been based on rheographic or impedographic measurement of the electrical impedance of tissue. In particular, numerous electrographic measurement techniques of both the active and passive type have been developed which measure the subsonic (1 Hz-20 Hz) and audio band (20 Hz-20 Hz) frequency alternating voltages associated with alterations in transmembrane or transmembrane-derived potentials. Electro-encephalography and electrocardiography are two common examples of such measurement techniques. However, the extremely long wavelength of the electrical events being measured prevents formation of images due to poor spatial resolution.
Previously, noninvasive imagery of biological targets using electromagnetic energy has been limited to X-rays. This region of the electromagnetic spectrum is very well suited to imagery due to the high photon energy and short wavelength characteristics of such radiation. However, the biological relevance of the imagery produced thereby is limited because the image formation mechanisms are due to photoelectric absorption and Compton scattering, which are events at the level of electronic and atomic states, rather than at the level of molecular states. Consequently, X-ray imagery finds its greatest utility in discriminating large density differences, such as those occuring at bone and air based interfaces. Although unenhanced density differences are just able to be visualized within soft tissues, the prime source of information is based on structure. Thus, disease is often not detectable using X-ray imagery until it has progressed to the point of gross structural-anatomical alteration.
Imaging using higher frequency impedographic measurement techniques has also been investigated. For example, applicants have determined that HF band (1 MHz-20 MHz) dispersion is sensitive to interfaces at the cell membrane (see applicant's article in IEEE Transactions on Microwave Theory & Techniques, MTT-26: 581 (1978) (hereinafter cited as IEEE Transactions 1978); and applicant's copending application, Ser. No. 938,625, entitled "An Electromagnetic Method for the Noninvasive Analysis of Cell Membrane Physiology and Pharmacology", filed on Aug. 31, 1978, which are both incorporated herein by reference.
HF band frequencies are theoretically more suitable for image formation than the subsonic and audio band frequencies traditionally used for pathologic study since HF band frequencies are a factor of approximately 10.sup.6 times higher in frequency. However, applicants are not aware of any apparatus which is presently available that can produce facsimile images based upon electric field constitutive parameters using HF band interrogating radiation.
The most advantageous frequencies with respect to the use of electric field constitutive parameters for the purpose of image formation on the basis of functional status are those in the microwave range (1 GHz-10 GHz). Microwave imagery is formed on the basis of the spatial distribution of the complex permittivity within the interrogated target and is biologically relevant because the complex permittivity of tissue is a quantity which has been shown to discriminate various classes of tissues as tabulated by Schwan (Table V in Chapter 6 of Physical Techniques in Biological Research, edited by W. K. Natuk, Vol. VI B). Furthermore, the complex permittivity and its dispersion as applied to tissue has been shown to discriminate physiological and pathological states as shown by Burdette et al. (U.S. Army Research Office, Project A-1755, Final Report for DAAG29-75-G-0182, 1979); by Cain et al. (ARO A-1755, Annual Report 1977, DAAG-75-G-0182); and by Larsen et al. (IEEE Transactions 1978).
Microwave imagery is also uniquely advantageous as a physiological imaging technique because of the fact that water is not only physiologically important in the study of biological dielectrics, but also is the chief determinant, along with electrolytes, of the microwave propagation properties of tissue. Thus, dispersion of microwave radiation may be related to many pathophysiological processes whereby complex permittivity is altered by water content, water distribution and water state. This explicitly includes the role of local vasculature in the dispersion of permittivity as well as the temporal and spatial variations of complex permittivity in tissue and organs. Moreover, dissipation of microwave energy within a target is relatable to the prediction of microwave hazards for specific target organs.
However, despite its unique biological relevance, microwave radiation heretofore has received little attention as a modality of imagery since the decimeter wavelengths necessary for penetration of thick targets composed of water dominated dielectrics have been previously considered to be incompatible with the resolution required for medical and scientific use, and because of the difficulties associated with processing the dispersion data which is obtained into an intelligible image.